An experimental study of adhesion and conformational transitions in polyelectrolytes.

We revisit the pH-induced conformational structure of poly(methacrylic) acid (PMAA)
chains in dilute solution using a combination of fluorescence techniques: nonradiative
energy transfer, fluorescence lifetime, and time-resolved anisotropy. These
measurements are supplemented with dynamic light scattering and transmission
electron microscopy experiments. The fluorescence lifetime results suggest that the
PMAA has a more complex structure than previously considered, with graded
changes in density in the collapsed state (low pH). Such structure also suggests that
the swelling of these dilute chains is of a progressive nature, with the outermost
parts responding to the changing pH before the central regions, rather than a
simultaneous swelling.
We have used neutron reflectometry to characterize the volume fraction-depth
profiles of poly[(diethyl amino)ethyl methacrylate] (PDEAMA) and poly(methacrylic)
acid (PMAA) brushes in aqueous solution as a function of pH. The polymers were
synthesized by atom transfer radical polymerization in order to create a dense brush
layer. In the case of PDEAMA .the brushes are collapsed at high pH, and swell at a
pH of between 3 and 4. The depth profiles of swollen brushes have an unusual
shape, with depletion in brush concentration close to the substrate, which increases
away from the substrate. Such a profile is unexpected for weak polyelectrolytes and
we propose that entanglements created during the growth of the brush are an
important consideration, creating a long-lived metastable equilibrium.
The PMAA brush was exposed to a pH cycle, showing a collapsed and expanded
brush at low and high pH values respectively.
In addition, we introduce and test here a new technique as a means to measure
interactions between polymer networks and brushes when immersed in aqueous
solution. The important feature offered by this technique is that measurements are
performed in situ. We have performed measurements showing that there is a
difference between the behaviour of PMAA gel/blank silicon and PMAA gel/brush
systems. These early results show interactions occurring at the interface between
gel and brush, which may be due to adhesion.